Many audio signals may be classified as having more speech like characteristics or more generic audio characteristics typical of music, tones, background noise, reverberant speech, etc. Codecs based on source-filter models that are suitable for processing speech signals do not process generic audio signals as effectively. Such codecs include Linear Predictive Coding (LPC) codecs like Code Excited Linear Prediction (CELP) coders. Speech coders tend to process speech signals well even at low bit rates. Conversely, generic audio processing systems such as frequency domain transform codecs do not process speech signals very well. It is well known to provide a classifier or discriminator to determine, on a frame-by-frame basis, whether an audio signal is more or less speech-like and to direct the signal to either a speech codec or a generic audio codec based on the classification. An audio signal processor capable of processing different signal types is sometimes referred to as a hybrid core codec. In some cases the hybrid codec may be variable rate, i.e., it may code different types of frames at different bit rates. For example, the generic audio frames which are coded using the transform domain are coded at higher bit rates and the speech-like frames are coded at lower bit rates.
The transitioning between the processing of generic audio frames and speech frames using speech and generic audio mode, respectively, is known to produce discontinuities. Transition from a CELP domain frame to a Transform domain frame has been shown to produce discontinuity in the form of an audio gap. The transition from transform domain to CELP domain results in audible discontinuities which have an adverse effect on the audio quality. The main reason for the discontinuity is the improper initialization of the various states of the CELP codec.
To circumvent this issue of state update, prior art codecs such as AMRWB+ and EVRCWB use LPC analysis even in the audio mode and code the residual in the transform domain. The synthesized output is generated by passing the time domain residual obtained using the inverse transform through a LPC synthesis filter. This process by itself generates the LPC synthesis filter state and the ACB excitation state. However, the generic audio signals typically do not conform to the LPC model and hence spending bits on the LPC quantization may result in loss of performance for the generic audio signals. Therefore a need exists for an encoder and decoder for processing an audio signal including generic audio and speech frames that improves audio quality during transitions between coding and decoding techniques.
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